This invention relates generally to atomic or molecular maser devices and more particularly to improvements in resonators, especially for a hydrogen maser frequency standard.
An atomic frequency standard is a device having a basic resonant system derived from an atomic or molecular specie experiencing a transition between two well-defined energy levels of the atom or molecule. The transition occurs in a reasonably convenient domain of the electromagnetic spectrum, the microwave region. The transition is employed as a highly stable frequency reference to which the frequency of a voltage-controlled crystal oscillator (VCXO) can be electronically locked. Thus, the high stability and relative insensitivity associated with an atomic reference frequency is thereby transferred to the VCXO.
Hydrogen, cesium and rubidium frequency standards are atomic-controlled oscillators in which the frequency of usually a 5 MHz or 10 MHz quartz crystal oscillator is controlled by means of a physics package and associated electronics that are devoted to maintaining that assigned output on a very long-term, exceedingly accurate and stable basis. By properly slaving the quartz crystal oscillator to the frequency of the atomic transition, the tendency of the quartz crystal to exhibit drifting due to aging and other inherent as well as environmental effects is markedly suppressed.
In a hydrogen maser frequency standard, hydrogen is customarily stored under pressure in a container and is passed through and purified by a hot palladium-silver alloy diffusion barrier that provides control of the hydrogen flux and subsequent beam intensity. The hydrogen, in molecular form, is introduced into an electrical discharge defined by a dissociation chamber which dissociates diatomic hydrogen into atomic hydrogen. Thereafter, the resultant hydrogen atoms emerge in a low-pressure region, are collimated into a beam and directed or focused through a state selection device, usually having a passageway of a few millimeters in diameter. The state selection device may be a hexapole or quadrapole magnet having a magnetic flux density at the pole tips of generally about 0.7 T or more. The state selection device generates an inhomogeneous magnetic field and is designed in such a manner so as to withdraw atoms in the lower hyperfine energy state and allows those hydrogen atoms to pass having the upper hyperfine state into a high Q-cavity resonator located in a microwave field region. In the cavity resonator, the atoms of hydrogen undergo interaction with a microwave field at a resonant frequency of about 1.4 GHz.
The cavity resonator into which the selected atoms of hydrogen are directed is surrounded by magnetic shields and is provided with an inner solenoid that creates a weak, substantially uniform magnetic field which is applied to the microwave field region in order to separate the different sublevels of the hyperfine state to insure that transitions occur only between levels where the Zeeman effect is quadratic.
The cavity resonator is designed to allow extremely long interaction times of atoms with the microwave field by coating the walls of the cavity resonator with special compositions, generally a fluorocarbon or the like to reduce atom perturbations through wall collisions. The interaction with the microwave field induces the hydrogen atoms in the selected energy state to make a transition to the lower state, thus radiating energy of a precise frequency to the field. In an active maser system, maser oscillations are self-sustaining when the atom density in the cavity resonator is such that the resultant induced transitions radiate enough coherent energy to more than offset cavity losses.
The hydrogen that departs from the cavity resonator may be readily removed by a vacuum system, generally by means of a high speed, getter pump along with a titanium sputter ion pump, both of which are generally housed in separate vacuum chambers. The getter materials that may be used for such a pump include Zr-Al, Zr-C and Zr-V alloys.
Although the general conditions recited above relate to an active maser mode, viz., using the maser principle itself in which there is coherent stimulated emission of the radiation within a suitable resonant structure, there is another mode, the so-called passive mode which may be employed for standard frequency generators. In the passive mode, an ensemble of particles (i.e. atoms or molecules) undergoing the desired quantum transition is used as a resonator and an auxiliary source of radiation (slave oscillator) is employed to produce the transitions which occur when the frequency of the radiation is near the nominal frequency. Thus, in a passive frequency standard, the atomic resonance has to be probed by an electromagnetic signal at the proper frequency which is produced by an auxiliary frequency generator whereas in an active frequency standard there is self-sustaining oscillation at the atomic resonance frequency.